The long-term objective of the current application is to study the regulation of chromosomal synapsis in mice by novel protein complexes recently identified in our laboratory. Abnormality in meiosis is a leading cause of birth defects and infertility in humans. Chromosomal synapsis is facilitated by the attachment of meiotic chromosome ends (telomeres) to the nuclear envelope and by rapid movements of meiotic chromosomes during early prophase I. While actin cytoskeleton is essential for dynamic movements of meiotic chromosomes in budding yeast, the nature of the actin-dependent motive force responsible for chromosome movement in any organism is not yet understood. We previously identified Tex19 as a germ cell-specific gene in mice that encodes a small protein (351 aa) of unknown function. Now we have found that TEX19 regulates chromosomal synapsis. We have also found that TEX19 forms a tight complex with ubiquitin E3 ligase UBR2, an ubiquitin E3 ligase of the N-end rule proteolysis pathway, and actin motor protein myosin II in the testis. Therefore, we hypothesize that TEX19 modulates meiotic chromosome movement and chromosomal synapsis through its association with ubiquitin E3 ligase UBR2 and the actin motor protein myosin II. We propose to test this hypothesis through the following experimental strategies. In Specific Aim 1, we will investigate the molecular mechanisms by which TEX19-UBR2 and TEX19-myosin II complexes regulate chromosomal synapsis. In Specific Aim 2, we plan to elucidate the role of the actin motor protein myosin II and actin filaments in meiotic chromosomal synapsis and chromosome movements through genetic and time-lapse imaging analyses. An innovative combination of biochemical, genetic, cell biological, and time-lapse imaging approaches will provide insights into the mechanisms underlying the regulation of meiotic chromosome movements in mice, which has remained largely unexplored.